The present invention relates to a variable light distribution type headlamp, particularly an automotive headlamp, having a lens drive device for variably controlling the degree of convergence (amount of diffusion) and the angle of the irradiated beam.
A lighting apparatus of the type shown in FIGS. 1(a) and 1(b) has conventionally been employed for controlling the light distribution pattern of the irradiated beam of an automotive headlamp. As shown in these figures, the apparatus includes projector lamp 50 for emitting a parallel beam, a movable lens 54, and a fixed lens 55 disposed on the front optical axis L of the projector lamp 50 and movable lens 54. The projector lamp is composed of a light source bulb 51, a collimator lens 52, and a reflector 53 having an elliptical reflecting surface. The movable lens 54 is driven in the direction of arrows X or Y so as to control the light distribution pattern by converging/diffusing or inclining the beams in consideration of the positional relation between the lenses.
In the automotive headlamp of this type, however, there has been employed a combination of two lenses to reduce the length of the lighting apparatus, namely, a condensing lens (of focal length f1) as the movable lens 54 and a cylindrical concave lens (of focal length f2) as the fixed lens 55. As shown in FIG. 1(a), when the focal points F1 and F2 of the two lenses coincide at a position where the movable lens 54 and the fixed lens 55 are spaced apart from one another, a parallel irradiated beam is produced, thereby forming a spot-type distribution pattern. As shown in FIG. 1(b), when the focal points F1 and F2 of the two lenses are shifted apart by disposing the movable lens 54 close to the fixed lens 55, a diffused light distribution pattern is obtained.
With the arrangement described above, however, since the movable lens 54 is separated from the fixed lens 55 by a distance L1 when the spot pattern of FIG. 1(a) is obtained, the width W1 of the irradiated beam decreases as the beam incident on the fixed lens 55 is converged to a small beam diameter, which makes it difficult to control the light distribution pattern of the irradiated beam by means of the fixed lens 55. Thus, pattern irregularities are likely to occur.
Moreover, when the diffused light distribution pattern of FIG. 1(b) is selected, because the aspherical surface of the concave lens used as the fixed lens 55 greatly varies in shape in the central portion of the lens and varies less in the peripheral portion thereof, even though the movable lens 54 is set closer to the fixed lens, the range of diffusion may not be increased sufficiently.
Moreover, the conventional variable distribution type headlamp requires a complicated lens drive mechanism, making the overall lamp assembly large in size. Such headlamps cannot, as a practical matter, be used in vehicles where there is only a limited amount of space for the installation of the headlamp.